The present invention relates to a motor-driven brake system for generating a braking force by means of a torque of a motor. More specifically, the present invention relates to a motor-driven brake system provided with a parking brake function.
A motor-driven brake system of this type comprises a brake body including a caliper having provided therein a piston, a motor and a rotary-linear motion conversion mechanism for enabling rotation of the motor to be converted to a linear motion and transmitted to the piston. Thrust is imparted to the piston according to a rotation of a rotor of the motor, to thereby press a brake pad against a disk rotor, thus braking a wheel under a pressing force (generated braking force). Normally, in such a motor-driven brake system, a force applied by a driver to a brake pedal, or a stroke of the brake pedal, is detected by a sensor, and rotation (an angle of rotation) of the motor is controlled based on a value detected by the sensor, to thereby obtain a desired braking force.
Recently, various attempts have been made to increase the advantage provided by such a motor-driven brake system by providing the system with a parking brake function.
A motor-driven disk brake is reversible relative to a reactive force imposed by brake pads. Therefore, to apply a parking brake using such a motor-driven disk brake, it is necessary to provide a means to fix a piston.
For example, in a motor-driven caliper in which a rotary motion of the motor is converted to a linear motion, a rotor of the motor is adapted to be locked (a pressing force is maintained) by means of a solenoid actuator (hereinafter referred to simply as “solenoid”). To use the caliper for a parking brake, the rotor of the motor should be locked while maintaining the solenoid in a non-energized condition. To this end, use is made of (1) a mechanism for unlocking the rotor under normal braking by placing the solenoid in an energized condition, and for locking the rotor during use of the parking brake by placing the solenoid in a non-energized condition; or (2) a mechanism using a solenoid having a latch mechanism, in which the solenoid is temporarily energized in a direction for unlocking for normal braking, and is temporarily energized in a direction for locking for applying the parking brake (a mechanism for maintaining a pressing force).
An example of a motor-driven brake system of the above-mentioned type having a parking brake function is disclosed in Japanese Patent Application Public Disclosure No. 2003-42199.
The motor-driven brake system of Japanese Patent Application Public Disclosure No. 2003-42199 comprises a brake body including a caliper having provided therein a piston, a motor and a rotary-linear motion conversion mechanism for enabling rotation of the motor to be converted to a linear motion and transmitted to the piston. Thrust is imparted to the piston according to a rotation of a rotor of the motor, to thereby press a brake pad against a disk rotor, thus generating a braking force. The motor-driven brake system further comprises a parking brake lock mechanism (a pressing force maintaining mechanism). The parking brake lock mechanism comprises a ratchet wheel which is provided in the rotor of the motor. The parking brake lock mechanism comprises a plurality of substantially projecting tooth portions arranged continuously in a circumferential direction on an outer circumferential surface of the ratchet wheel. The parking brake lock mechanism further comprises an engaging pawl provided at a periphery of the ratchet wheel and adapted to move so as to be capable of being engaged with, or disengaged from, the ratchet wheel. The parking brake lock mechanism further comprises a solenoid (an actuator) for moving the engaging pawl, and applies the parking brake by engaging the engaging pawl with the ratchet wheel.
To apply the parking brake, the rotor of the motor is rotated in one direction to thereby generate a braking force. The solenoid is excited simultaneously with generation of the braking force, to thereby move the engaging pawl towards the ratchet wheel for engagement. Due to this engagement between the engaging pawl and the ratchet wheel, rotation of the rotor in the direction for releasing the brake is restricted, thus exerting a parking brake function.
If the parking brake is actuated when the brake pads are at a high temperature, the pressing force of the brake pads changes with time during application of the parking brake, as a result of contraction of the brake pads as their temperature lowers. FIG. 10 shows how the pressing force of the brake pads changes during application of the parking brake. As indicated by a curved line A in FIG. 10, the pressing force is gradually reduced with time below the pressing force that is required for performing a parking brake function (hereinafter frequently referred to simply as “the required pressing force”). In the motor-driven brake system, a desired pressing force can be generated. Therefore, to obviate the above-mentioned problem, a pressing force generated at a time when the parking brake is actuated (for convenience sake, hereinafter frequently referred to as “the pressing force when the parking brake is actuated”) can be set to be large, allowing for a lowering of the pressing force due to a lowering of temperature of the brake pads. However, to generate a large pressing force at a time the parking brake is actuated, the caliper must be increased in size, so as to ensure its durability.
To avoid such an increase in size of the caliper, it has been proposed to use a motor-driven brake system in which, as indicated by a curved line B in FIG. 10, the parking brake is first operated to thereby generate a pressing force that is slightly higher than the required pressing force, and thereafter the parking brake is re-operated a predetermined number of times at predetermined time intervals or, while monitoring a lowering of the pressing force using a pressing force detecting means, re-operation of the parking brake (involving continuous checking of the pressing force by means of the pressing force detecting means, driving of the motor and driving of the solenoid of the parking brake lock mechanism) is conducted each time the pressing force reaches the required pressing force (see Japanese Patent Public Disclosure No. 2002-225701).
However, in the former system (the motor-driven brake system in which the parking brake is re-operated at predetermined time intervals), it is required to actuate the parking brake a predetermined number of times at predetermined time intervals even after the engine is stopped. In this case, when the parking brake is actuated many times, the power supply of the motor-driven brake system cannot be turned off for a long time period, which leads to a high consumption of power. On the contrary, if the number of times the parking brake is actuated is reduced, a lowering of the pressing force due to a lowering of temperature of the brake pads is likely to occur. Therefore, this motor-driven brake system is still unsatisfactory.
Further, in the latter system (re-operation of the parking brake is conducted each time the pressing force reaches the required pressing force), it is required to continuously check whether a pressing force reaches the required pressing force, by means of the pressing force detecting means. Therefore, power is consumed over a long time period. Therefore, as in the case of the former system, this motor-driven brake system is also still unsatisfactory.